.. include:: ../references.txt .. _detect: ************************* detect - Source detection ************************* .. currentmodule:: gammapy.detect Introduction ============ The `gammapy.detect` submodule includes low level functions to compute significance and test statistics images as well as some high level source detection method prototypes. Detailed description of the methods can be found in [Stewart2009]_ and [LiMa1983]_. Note that in Gammapy maps are stored as Numpy arrays, which implies that it's very easy to use `scikit-image`_ or `photutils`_ or other packages that have advanced image analysis and source detection methods readily available. Computation of TS images ======================== .. gp-image:: detect/fermi_ts_image.png :width: 100% Test statistics image computed using `~gammapy.detect.TSMapEstimator` for an example Fermi dataset. The `gammapy.detect` module includes a high performance `~gammapy.detect.TSMapEstimator` class to compute test statistics (TS) images for gamma-ray data. The implementation is based on the method described in [Stewart2009]_. Assuming a certain source morphology, which can be defined by any `astropy.convolution.Kernel2D` instance, the amplitude of the morphology model is fitted at every pixel of the input data using a Poisson maximum likelihood procedure. As input data a counts, background and exposure images have to be provided. Based on the best fit flux amplitude, the change in TS, compared to the null hypothesis is computed using `~gammapy.stats.cash` statistics. To optimize the performance of the code, the fitting procedure is simplified by finding roots of the derivative of the fit statistics with respect to the flux amplitude. This approach is described in detail in Appendix A of [Stewart2009]_. To further improve the performance, Pythons's `multiprocessing` facility is used. The following example shows how to compute a TS image for Fermi-LAT 3FHL galactic center data: .. code-block:: python from astropy.convolution import Gaussian2DKernel from gammapy.detect import TSMapEstimator from gammapy.maps import Map from gammapy.cube import PSFKernel counts = Map.read("$GAMMAPY_DATA/fermi-3fhl-gc/fermi-3fhl-gc-counts.fits.gz") background = Map.read("$GAMMAPY_DATA/fermi-3fhl-gc/fermi-3fhl-gc-background.fits.gz") exposure = Map.read("$GAMMAPY_DATA/fermi-3fhl-gc/fermi-3fhl-gc-exposure.fits.gz") maps = { "counts": counts, "background": background, "exposure": exposure, } kernel = PSFKernel.read("$GAMMAPY_DATA/fermi-3fhl-gc/fermi-3fhl-gc-psf.fits.gz") ts_estimator = TSMapEstimator() result = ts_estimator.run(maps, kernel.data) The function returns an dictionary, that bundles all resulting maps. E.g. here's how to find the largest TS value: .. code-block:: python import numpy as np np.nanmax(result['ts'].data) Computation of Li & Ma significance images ========================================== The method derived by [LiMa1983]_ is one of the standard methods to determine detection significances for gamma-ray sources. Using the same prepared Fermi dataset as above, the corresponding images can be computed using the `~gammapy.detect.compute_lima_image` function: .. code-block:: python from astropy.convolution import Tophat2DKernel from gammapy.maps import Map from gammapy.detect import compute_lima_image counts = Map.read("$GAMMAPY_DATA/fermi-3fhl-gc/fermi-3fhl-gc-counts.fits.gz") background = Map.read("$GAMMAPY_DATA/fermi-3fhl-gc/fermi-3fhl-gc-background.fits.gz") kernel = Tophat2DKernel(5) result = compute_lima_image(counts, background, kernel) The function returns a dictionary, that bundles all resulting images such as significance, flux and correlated counts and excess images. Using `gammapy.detect` ====================== :ref:`tutorials` that show examples using ``gammapy.detect``: * :gp-notebook:`detect_ts` Reference/API ============= .. automodapi:: gammapy.detect :no-inheritance-diagram: :include-all-objects: